Trends in Phosphorus Source Pressure at Field to Watershed Scale- a Tale of Two Arable Watersheds.

In two hydrological contrasting Irish arable watersheds the aim of this research was to evaluate the effectiveness of phosphorus (P) policy measures via the EU Nitrates Directive, to decrease soils with excessive soil P, across a four year study period. In both watersheds (Arable A - 11.2km² and Arable B- 9.5km²) the soils were surveyed to a 10cm depth at a ≤2 ha resolution for plant available P (Morgan’s P) during 2009-2013 and 2010-2014. Based on additional grid-sampling the soils in each watershed were further characterized for their P mobilisation potential (e.g. aluminium concentrations). During this study period (2010-2013), annual field P nutrient input and off-take records were collected and used to calculate field P balances. Simultaneously, water discharge and P concentration data were captured using high temporal resolution monitoring. Field P balances in Arable A showed an average surplus P ranged from 1.9 kg/ha/year in 2011 to 7.5 kg/ha/year in 2013. However, between this same period, excessive soil P concentrations decreased (20 % to 12 %), indicating that high aluminium concentrations in these soils bind P inputs and decreased P availability. Similarly, between 2010 and 2013, in Arable B the average surplus field P increased from -0.42 to 25.5 kg/ha/year, however the excessive soil P concentrations increased (22 % to 26 %). In part, this increase is attributed to some fields receiving excess applications of organic nutrient forms above crop requirements. Subsequent, P concentrations measured at outlet of these watersheds were lower and for most of the time below the Irish environmental quality standard threshold in Arable A compared to the Arable B watershed. This study highlights the need to consider the P buffering and mobilisation potential between soil types and how future nutrient management regulations should consider these watershed characteristics when expecting improvements in national water quality status.